Till the moment I have been using 125kHz for the radio frequency reading of the cards. That means normally 35 turns of copper and also a big area around the Plobot that any other Plobot is around it.

Imagine the scenario, two kids happily playing in different corners of the room, each one programming their Plobot independently. But at some moment one is close to the other and without any apparent reason it stops responding.

The solution has not been found yet, but at least the first step was to reduce the size of the antenna, not only from 35 turns to 4 but also in diameter, from 8cm to almost 4cm. It was a proper hack into a standard RC522 module, but it works !